Get It Right Every Time with Pre-Compliance Testing

A big question that you might have is, what is the difference between Pre-Compliance and Compliance testing? What makes Pre-Compliance important? Pre-Compliance is a low risk, cost effective method to ensure your DUT will pass final Compliance testing. Waiting until the end of a product development cycle for compliance testing is risky due to its high cost. The cost includes reserving time in a compliance test lab, and the cost of redesigning your DUT if compliance testing does not pass. Reserving time in the lab can be difficult, as these labs are in high demand. It may be a long period of time before the lab is available, which could mean launch delays. These are all unexpected expenses, in addition to expensive test time.

What is EMC Compliance Testing?

Electromagnetic Compatibility (EMC) Testing, is the interaction of electrical equipment with its electromagnetic environment, and other equipment. All electronic devices have the potential to emit electromagnetic fields, and compliance testing is the final stage of testing that ensures the electronic devices operate safely.

An example of poor compliance is when your TV picture quality is wrecked by wavy interference lines each time you turn on the kitchen blender. This is something that shouldn’t happen – you should be able to have your TV operate normally, regardless of a kitchen blender running. The blender produces electromagnetic waves that interfere with the TV signal. Electronics need proper shielding to avoid interference with other devices. This example is quite harmless, but if you think about unintentional electromagnetic interference on a larger scale, it could be a safety hazard, for example, it could corrupt data.

Figure 1: An anechoic chamber is required for compliance testing

Common Measurements for Compliance

Developing your own EMC test lab will help you check your designs for compliance while they are in development and undergoing revisions. Verifying your designs on your own is called pre-compliance testing. Pre-compliance testing closely simulates the way compliance test s are run – putting your designs to test against actual test limits. Once you are confident in your design, you can take it to a third party lab for final compliance testing. This will make your testing more efficient and cost effective than relying on limited external laboratory tests.

The top most common compliance test failures are:

Radiated Emissions

Radiated Immunity

Conducted Emissions

Electrostatic Discharge (ESD)

Let’s go over what each of these are:

Radiated emissions testing

Radiated emission testing measures the radiated E-fields emanating from the DUT, and is usually the most common test failure. All devices will have some amount of emissions, but as long as they meet the requirements of your standards body you will be compliant. A radiated emissions test involves measuring a DUT’s radiated emissions using a signal analyzer and an antenna.

Radiated immunity testing

The next common point of failure is radiated immunity. Radiated immunity is a measure of how much external electric fields from external sources the CUT can tolerate before its performance starts to degrade. This test set up requires 3 signal generators to cover the entire frequency range, RF broadband power amplifiers, and 2 – 3 antennas.

Figure 3: Demonstration of the difference of Emissions and Immunity / Susceptibility

Conducted emissions testing

Conducted emissions testing focuses on the unwanted signals the DUT generates on the AC mains. Both radiated and conducted testing are very important as you will not pass compliance testing if either of these fails. For conducted emissions testing, your will need a spectrum analyzer equipped with EMC pre-compliance measurement software, line impedance stabilization network (LISN), and a limiter. For more info refer back to the blog, "Complete your EMC conducted emissions testing in just 7 steps".

Figure 2: Sample set-up of a radiated emissions measurement

Electrostatic Discharge testing

ESD shield testing checks how immune the DUT is to static discharges, usually from operators touching key pads or touchscreens. The set up for ESD testing requires a ground plane that the DUT is connected to and several sheets of metal of various thickness to observe how the DUT interacts with the various planes, thickness and materials.

Proper pre-compliance testing is crucial if you want to avoid surprises during compliance testing. By checking your designs for electromagnetic compatibility during your design and verification work, you can ensure that you will pass compliance testing on the first try. If you fail compliance testing, there’s a best-case scenario. A simple design tweak fixes the issue and your only added cost is more time with the compliance lab. As you probably know, the best-case scenario rarely happens. Many times a failed compliance test means you have to do a significant design rework, which can mean a delayed product ship date.